A conventional radio receiver, which is applied to base station radio device, is as shown in FIG. 1 composed of an antenna terminal 51, a receive band pass filter 52 that selects a desired band signal from receive signal to be inputted from the antenna terminal 51, a receive low noise amplifier 53 that amplifies the output of the receive band pass filter 52 with low noise up to a desired level, a receive signal output terminal 54 that outputs the receive signal amplified by the receive low noise amplifier 53, a vacuum container 55 that seals in vacuum the receive band pass filter 52 and the receive low noise amplifier 53, a cooling means 56 that cools the interior of the vacuum container 55 through a cooling stage 56a, a first power supply terminal 57 for supplying operating power to the receive low noise amplifier 53, a second power supply terminal 58 for supplying operating power to the cooling means 56, and a housing 59 that houses the vacuum container 55 and the cooling means 56.
The receive band pass filter 52 and the receive low noise amplifier 53 are composing a signal processing part (high-frequency receiving part) and are sealed in vacuum in the vacuum container 55, thereby insulated in vacuum from outside, and are cooled by the cooling means 56.
The inside of the vacuum container 55 is kept in vacuum and insulated in vacuum from outside, and the invasion of heat from outside is thereby blocked out. The receive band pass filter 52 and the receive low noise amplifier 53, which are sealed in vacuum inside the vacuum container 55, are cooled to an extremely low temperature, e.g., about 70 K (nearly equal to −203° C.) by the cooling means 56. The cooling means 6 is, for example, composed of a freezing machine such that can stably keep an extremely low temperature of e.g. 70 K for a long period by a heat exchange cycle using compression and expansion of, e.g., helium gas, and it may be a commercially-available apparatus.
Thus, by cooling the receive band pass filter 52 and the receive low noise amplifier 53 to an extremely low temperature, heat noise occurred in the receive band pass filter 52 and the receive low noise amplifier 53 can be reduced. As a result, noise figure of the radio receiver in FIG. 1 can be significantly reduced and the sensitivity thereof can be significantly improved. Therefore, in this radio receiver, receive output with a regulation C/N (carrier wave/noise power) can be obtained even for a receive signal with low level and transmit power on the transmission side required to get the regulation C/N can be reduced. Such a radio receiver is frequently placed in the air or near the top of antenna tower so as to reduce the loss until inputted to the antenna terminal 51.
The interior of the vacuum container 55 is as described above needed to be kept in vacuum to insulate that from the outside. In the case of radio receiver placed within doors, the interior of the vacuum container 55 is always evacuated by using a vacuum pump. However, in the case that the radio receiver is placed in the air or near the top of antenna tower to be used as a base station device for mobile communication, it is necessary that the radio receiver is miniaturized and lightened to make its installation easy. Thus, it is not desirable that the radio receiver has an external vacuum pump for evacuating always the interior of the vacuum container 55 since it becomes difficult to miniaturize and lighten. Taking the maintenance after installation into account, since the external pump needs a periodical maintenance, the interval of maintenance has to be shortened. This is not desirable since it increases the maintenance cost on the communication service. Therefore, conventionally, in keeping the interior of the vacuum container 55 in vacuum, a vacuum pump is connected to the vacuum container 55 from outside and the vacuum container 55 is sealed up when it becomes evacuated enough for vacuum insulation.
However, in the conventional radio receiver, even after the vacuum container 55 is sealed up when it becomes evacuated enough for vacuum insulation, the gas pressure inside the vacuum container 55 increases since the vacuum insulation degrades with time during a long operation time. Therefore, it is necessary for the interior of the vacuum container 55 to be evacuated periodically and the maintenance cost increase by that much.
This is caused by gas to be discharged from the receive band pass filter 52 and the receive low noise amplifier 53 being sealed in vacuum in the vacuum container 55. Namely, the receive band pass filter 52 and the receive low noise amplifier 53 discharges unnecessary gas such as oxygen, nitrogen and hydrogen with time during the long operation time. When 1000 hours elapse, vacuum insulation in the vacuum container 55 degrades and the cooling efficiency by the cooling means 56 is reduced. As a result, it is necessary for the interior of the vacuum container 55 to be evacuated periodically after about 1000 hour operation and, therefore, the maintenance cost increase by that much. Meanwhile, gas discharged from the inside wall of the vacuum container 55 can be suppressed to a extremely low level by using a ultrahigh vacuum material such as aluminum alloy or electropolishing or passivating the inside wall.